Thromboembolic events are the most common cause of mortality in patients with paroxysmal nocturnal hemoglobinuria (PNH). Low plasma heparin cofactor II (HCII) levels have been shown to occur in a variety of hemolytic conditions including thalassemia intermedia and sickle cell disease. The level of HCII is related to the degree of hemolysis. A correlation between low HCII levels and thrombosis has been demonstrated in some of these patients. Because of the association between PNH and thrombosis, we are exploring the possible association between HCII levels and PNH. We have already demonstrated that PNH patients have low normal baseline levels of HCII. We will now focus our efforts on measuring HCII in patients with a significant history of thrombosis. Other coagulation parameters including ATIII, Prot C, Prot S, APCR, Prothrombin 20210, and Methylenetetrahydrofolate polymorphisms will also be measured.